Extract 2 words in any order

Question

I would like to extract cat and dog in any order

string1 <- "aasdfadsf cat asdfadsf dog"
string2 <- "asfdadsfads dog asdfasdfadsf cat"

What I have now extracts cat and dog, but also the text in-between

stringr::str_extract(string1, "cat.*dog|dog.*cat"

I would like the output to be

cat dog

and

dog cat

for string1 and string2, respectively

Answer 1

You may use sub with the following PCRE regex:

.*(?|(dog).*(cat)|(cat).*(dog)).*

See the regex demo .

Details

• .* - any 0+ chars other than line break chars (to match all chars add (?s) at the pattern start)
• (?|(dog).*(cat)|(cat).*(dog)) - a branch reset group (?|...|...) matching either of the two alternatives:
  • (dog).*(cat) - Group 1 capturing dog , then any 0+ chars as many as possible, and Group 2 capturing cat
  • | - or
  • (cat).*(dog) - Group 1 capturing cat , then any 0+ chars as many as possible, and Group 2 capturing dog (in a branch reset group, group IDs reset to the value before the group + 1)
• .* - any 0+ chars other than line break chars

The \\1 \\2 replacement pattern inserts Group 1 and Group 2 values into the resulting string (so that the result is just dog or cat , a space, and a cat or dog ).

See an R demo online , too:

x <- c("aasdfadsf cat asdfadsf dog", "asfdadsfads dog asdfasdfadsf cat")
sub(".*(?|(dog).*(cat)|(cat).*(dog)).*", "\\1 \\2", x, perl=TRUE)
## => [1] "cat dog" "dog cat"

To return NA in case of no match, use a regex to either match the specific pattern, or the whole string, and use it with gsubfn to apply custom replacement logic:

> gsubfn("^(?:.*((dog).*(giraffe)|(giraffe).*(dog)).*|.*)$", function(x,a,b,y,z,i) ifelse(nchar(x)>0, paste0(a,y," ",b,z), NA), x)
[1] "NA" "NA"
> gsubfn("^(?:.*((dog).*(cat)|(cat).*(dog)).*|.*)$", function(x,a,b,y,z,i) ifelse(nchar(x)>0, paste0(a,y," ",b,z), NA), x)
[1] "cat dog" "dog cat"

Here,

• ^ - start of the string anchor
• (?:.*((dog).*(cat)|(cat).*(dog)).*|.*) - a non-capturing group that matches either of the two alternatives: .*((dog).*(cat)|(cat).*(dog)).* :
  • .* - any 0+ chars as many as possible
  • ((dog).*(cat)|(cat).*(dog)) - a capturing group matching either of the two alternatives:
    • (dog).*(cat) - dog (Group 2, assigned to a variable), any 0+ chars as many as possible, and then cat (Group 3, assigned to b variable)
    • |
    • (cat).*(dog) - dog (Group 4, assigned to y variable), any 0+ chars as many as possible, and then cat (Group 5, assigned to z variable)
  • .* - any 0+ chars as many as possible
    • | - or
    • .* - any 0+ chars
• $ - end of the string anchor .

The x in the anonymous function represents the Group 1 value that is "technical" here, we check if the Group 1 match length is not zero with nchar , and if it is not empty we replace with the custom logic, and if the Group 1 is empty, we replace with NA .

Answer 2

We can use str_extract_all from the stringr package with capture groups.

string1 <- "aasdfadsf cat asdfadsf dog"
string2 <- "asfdadsfads dog asdfasdfadsf cat"
string3 <- "asfdadsfads asfdadsfadf"

library(stringr)
str_extract_all(c(string1, string2, string3), pattern = "(dog)|(cat)")
# [[1]]
# [1] "cat" "dog"
# 
# [[2]]
# [1] "dog" "cat"
# 
# [[3]]
# character(0)

We can also set simplify = TRUE . The output would be a matrix.

str_extract_all(c(string1, string2, string3), pattern = "(dog)|(cat)", simplify = TRUE)
#       [,1]  [,2] 
# [1,] "cat" "dog"
# [2,] "dog" "cat"
# [3,] ""    ""  

Answer 3

Or,

> regmatches(string1,gregexpr("cat|dog",string1))
[[1]]
[1] "cat" "dog"

> regmatches(string2,gregexpr("cat|dog",string2))
[[1]]
[1] "dog" "cat"